The present invention relates generally to a suspension control system for an automotive vehicle, which automatically controls damping characteristics or stiffness of a vehicular suspension depending upon vehicle driving conditions. More specifically, the invention relates to an automotive suspension control system employing a road-condition-dependent suspension control to ensure riding comfort of the vehicle even on an undulating road.
Various uses of road preview sensors have been proposed and developed. For example, SAE Technical Paper Series Nos. 630750 and 800520, respectively published on October, 1968 and February, 1980 show road preview sensors for use in suspension systems for obtaining optimum riding comfort and drivability. In addition, Japanese Patent First Publication No. 57-172808, published on Oct. 23, 1982 discloses a vehicle height control system which includes a sensor which detects rough road conditions and adjusts the vehicle height level depending upon road surface conditions. A vehicle height or level sensor is employed in the disclosed vehicle height control system for monitoring the relative displacement between the vehicle body and wheel axle. The output of the vehicle level sensor is compared with a reference level, which serves as a rough road criterion, and adjusts the vehicle height according to the recognized road surface conditions.
In another example, Japanese Patent First Publication No. 58-30542, published on Feb. 23, 1983, discloses a variable damping force shock absorber with damping characteristics varying in accordance with vehicle driving conditons. In the disclosed system, the magnitude of relative displacement between the vehicle body and wheel axle is measured and a vehicle height variation indicative signal is derived from the measured displacement and the instantaneous vehicle speed. The vehicle height variation indicative signal value is compared with a reference value which serves as a stiff suspension criterion for adjustment of the damping characteristics of the shock absorber in accordance therewith.
On the other hand, Published Japanese Patent Application (Tokkai) Sho. 56-42739 discloses a suspension control system which controls the damping force produced in response to rolling forces depending upon vehicle speed and steering adjustments. The damping force is so controlled as to be HIGH when the vehicle speed is higher than a set speed and the steering adjustments through greater than a predetermined angle are performed. In addition, the owner of the present application has already disclosed a suspension control system in Published Japanese Utility Model Application (Jikkai) Sho. 56-147107. In this system, the damping force produced by a suspension shock absorber is adjusted between HIGH and LOW levels depending upon the vehicle speed, the magnitude of steering adjustments and the angular velocity of steering adjustments.
In these prior art systems, road condition dependent suspension control has been performed with reference to a road surface condition indicative parameter. Such prior art suspension control systems achieve driving stability of the vehicle and riding comfort to some extent. However, in the prior art, it has been difficult to adjust damping characteristics or stiffness of the suspension in precise accordance with the road surface conditions.